## 21.19. Joining Lines with join

If you've worked with databases, you'll probably know what to do with the Unix join command; see your online manual page. If you don't have a database (as far as you know!), you still probably have a use for join: combining or "joining" two column-format files. join searches certain columns in the files; when it finds columns that match one another, it "glues the lines together" at that column. This is easiest to show with an example.

I needed to summarize the information in thousands of email messages under the MH mail system. MH made that easy: it has one command (scan) that gave me almost all the information I wanted about each message and also let me specify the format I needed. But I also had to use wc -l (Section 16.6) to count the number of lines in each message. I ended up with two files, one with scan output and the other with wc output. One field in both lines was the message number; I used sort (Section 22.1) to sort the files on that field. I used `awk '{print $1 "," $2}'` to massage wc output into comma-separated fields. Then I used join to "glue" the two lines together on the message-number field. (Next I fed the file to a PC running dBASE, but that's another story.)

Here's the file that I told scan to output. The columns (message number, email address, comment, name, and date sent) are separated with commas (,):

>     0001,andrewe@isc.uci.edu,,Andy Ernbaum,19901219
>     0002,bc3170x@cornell.bitnet,,Zoe Doan,19910104
>     0003,zcode!postman@uunet.uu.net,,Head Honcho,19910105
>        ...

Here's the file from wc and awk with the message number and number of lines:

>     0001,11
>     0002,5
>     0003,187
>        ...

The following join command then joined the two files at their first columns (`-t,` tells join that the fields are comma-separated):

>     % join -t, scanfile wcfile

The output file looked like this:

>     0001,andrewe@isc.uci.edu,,Andy Ernbaum,19901219,11
>     0002,bc3170x@cornell.bitnet,,Zoe Doan,19910104,5
>     0003,zcode!postman@uunet.uu.net,,Head Honcho,19910105,187
>        ...

Go to http://examples.oreilly.com/upt3 for more information on: join

join can do a lot more than this simple example shows. See your online manual page. The GNU version of join is on the CD-ROM [see http://examples.oreilly.com/upt3].

-- JP

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